The International Telecommunications Union (ITU) has proposed highly rigorous requirements for the performance of the next-generation mobile communication system of International Mobile Telecommunications-Advanced (IMT-A). For example, the maximum transmission bandwidth of the system shall be up to 100 MHz, peak rates of uplink and downlink data transmission shall be up to 1 Gbps and 500 Mbps, and there is a very high demand proposed for the average spectrum efficiency, particularly the edge spectrum efficiency, of the system. In order to satisfy the requirements on the new system of IMT-Advanced, the 3rd Generation Partnership Project (3GPP) has proposed in the next-generation mobile cellular communication system of LTE-Advanced (Long Term Evolution-Advanced) the use of multi-point coordinated transmission to improve the performance of the system. Multi-point coordinated transmission refers to coordination among a plurality of geographically separate transmission points, which typically are base stations of different cells. Multi-point coordinated transmission is divided into downlink coordinated transmission and uplink joint reception. Solutions of downlink multi-point coordinated transmission are generally divided into joint scheduling and joint transmission. Joint scheduling refers to coordination of time, frequency and space resources among the cells so as to allocate orthogonal resources to different User Equipments (UEs) to thereby avoid interference to each other. Inter-cell interference is a dominating factor restricting the performance of a UE at the edge of a cell, therefore, joint scheduling can reduce inter-cell interference to thereby improve the performance of the UE at the edge of the cell.
FIG. 1 is a schematic diagram of joint scheduling, and as illustrated, joint scheduling refers to coordination of time, frequency and space resources among cells so as to allocate orthogonal resources to different UEs to thereby avoid interference to each other. Inter-cell interference is a dominating factor restricting the performance of a UE at the edge of a cell, therefore, joint scheduling can reduce inter-cell interference to thereby improve the performance of the UE at the edge of the cell. As illustrated in FIG. 1, three UEs possibly interfering with each other can be scheduled onto orthogonal resources (different resources are represented with different types of lines in the figure) through joint scheduling of three cells to thereby effectively avoid interference among the cells.
FIG. 2 is a schematic diagram of joint transmission, and as illustrated, data is transmitted concurrently from a plurality of cells to a UE in the joint transmission solution to enhance signals received by the UE as opposed to the joint scheduling solution in which data is transmitted from only one cell to a UE. As illustrated in FIG. 2, data is transmitted from three cells to a UE over the same resource, and the UE receives signals of the cells concurrently. Superposition of useful signals from the cells can improve the quality of the signals received by the UE on one hand and reduce interference to the UE to thereby improve the performance of the system on the other hand.
FIG. 3 is a schematic diagram of uplink joint reception, and as illustrated, uplink joint reception refers to concurrent reception of data transmitted from a UE by a plurality of cells and joint processing of data received by the cells to improve the quality of demodulating the UE data.
In the LTE system, a UE transmits an uplink Sounding Reference Signal (SRS) over time and frequency resources specified by an anchor cell. A base station estimates, according to the SRS signal transmitted from the UE and a signal received by the base station, information of a channel from the UE to the base station as a criterion for uplink scheduling in the frequency domain, selection of Modulation and Coding Scheme (MCS) and allocation of a resource. For a Time Division Duplex (TDD) system, the base station may calculate a weighting vector of downlink beam forming according to acquired unlink channel information. An SRS resource in the LTE system is allocated per cell so that resources of UEs in a cell are orthogonal to each other and SRSs of adjacent cells may be configured to be transmitted in different sub-frames. Furthermore, a sequence transmitted from a UE is specified by a cell. Therefore, there is such a drawback in the prior art that channels of a plurality of cells can not be sounded concurrently in the LTE-A system.